It is common practice to use hydraulic power wrenches for tightening screws, said power wrenches comprising a piston-cylinder unit which reciprocatingly drives a ratchet lever. The ratchet lever drives a ring element which is coupled via a socket wrench with the screw to be turned. The rotating strokes of the ratchet lever in one direction effect tightening of the screw, while the return stroke of the ratchet lever is an idle stroke.
In the case of screws which are tightened by means of power wrenches an exact determined bias-tension must be obtained to ensure that the screw on the one hand tensions the associated bolt in a defined manner, and on the other hand does not over-tension the bolt. For attaining a defined tension it does not suffice to measure the hydraulic pressure of a hydraulic power wrench and to stop the screw process when the hydraulic pressure has reached a limit value. When screws are tightened, unexpected obstacles may be encountered, e.g. the screw may get stuck or canted due to defects of the thread or rust. The screw resistance is a suitable measure for realizing defined screw conditions.